This invention relates to stator assemblies for use in alternating current generators and electric motors, and more particularly, to a high-density stator core fabricated from an iron-based powder by electromagnetic pressing.
Alternating current generators and electric motors typically incorporate a fixed stator assembly for inductively coupling electrical energy from an adjacent rotating component or rotor. A stator assembly incorporates a magnetic stator core fabricated traditionally from thin laminates of an iron-based material such as a silicon-iron alloy. Individual laminations are punched from flat sheets of the ferrous material using specialized dies with the required shape and number of slots and teeth. The individual laminations are coated with a thin insulating layer to reduce eddy current losses, carefully aligned in a stack, and secured to form a stator core. Conductors are wound in the slots to complete the stator assembly for incorporation into a generator or motor.
Stator cores have also been produced from an iron-based powder using conventional powder metallurgy techniques. A near-net shape, single piece green compact is produced by applying a large uniaxial pressure to compress a quantity of the powder that is dimensionally confined within a die. The die possesses a geometric shape with features that complement the desired features of the stator core. Stator cores formed from pure iron powder by conventional powder metallurgy techniques typically have a density of about 7.2 g/cm3 to about 7.3 g/cm3. An example of a stator core fabricated from an iron-based powder by conventional powder metallurgy techniques is disclosed in U.S. Pat. No. 4,947,065 issued to Ward et al.
Iron-based powder is a magnetic material that is subject to undesirable hysteresis losses and eddy current losses when it is exposed to a rapidly varying electromagnetic field. Thus, prior to compaction, the iron-based powder is coated with a dielectric material using one of a number of well known processes. The dielectric coating electrically insulates individual particles of iron to minimize core losses due to eddy currents and hysteresis. Such coatings include thermoplastics, such as disclosed in U.S. Pat. No. 5,211,896 issued to Ward et al., iron phosphates, such as disclosed in U.S. Pat. No. 5,063,011 issued to Rutz et al., and alkali metal silicates, such as disclosed in U.S. Pat. No. 4,601,765 issued to Soileau et al.
In conventional powder metallurgy techniques, the compact may be sintered after compacting to develop metallurgical bonds by mass transfer under the influence of heat. However, subsequent thermal treatment of coated iron powder degrades the electrical insulating properties of the dielectric coating, particularly for a thermoplastic coating, and produces a stator core having unsatisfactory magnetic properties.
Stator cores of coated iron powder compacted by traditional powder metallurgy techniques have magnetic properties significantly inferior to those of a stator core constructed from stacked laminations, especially for low-frequency applications. More specifically, a stator core formed of iron-based powder will generally have a lower flux capacity, a reduced permeability, and higher hysteresis losses than a comparable laminated stator core. The inferiority in magnetic properties, particularly the induction permeability, is believed to be due in part to the inability of traditional powder metallurgy techniques to sufficiently densify the particles of coated iron.
Conventional laminated stator cores require many different operations in their manufacture. Although compaction of metal powder to fabricate a stator core eliminates the need to punch and stack individual laminations, significant drawbacks limit the utility of a stator core formed from metal powder. The tooling and equipment, including a die and a very high tonnage press, required for conventional powder metallurgy techniques is costly and can only be justified by the prospect of large volume production. In addition, parts having a complex geometry and a large surface area are difficult to fabricate with uniform material properties. For example, in a conventional stator core design, the poles that project radially from a yoke portion are difficult to densify.
A lubricant may be admixed with the iron-based powder before compaction. The lubricant reduces friction during pressing, which improves the compressibility of the iron-based powder and the uniformity of densification throughout the body of the part. However, the admixed lubricant reduces the strength of the compact by forming a residual lubricant film between the metal particles.
Specialized powder metallurgy processes have emerged that can compact a metal powder to a density approaching full density, such as pneumatic forging, hot isostatic pressing, and powder forging. However, cores for ignition coils fabricated from coated iron powder by such techniques have exhibited unacceptably high core losses due to thermal breakdown of the dielectric coating.
There is thus a need for a high-density stator core produced as a near-net-shape part from an iron-based powder and that has improved magnetic properties optimized for use in an alternating current generator or electric motor.
The present invention addresses these and other problems associated with the prior art by providing a stator core structure fabricated as a highly-dense, near-net-shape component from an iron-based powder. Further, the present invention provides a stator core having enhanced magnetic properties suitable to replace laminated stator cores for use in dynamoelectric devices, such as alternating current generators and motors. Further, the present invention provides a stator core formed from an iron-based powder coated with a dielectric material, wherein the stator core has magnetic properties superior to stator cores fabricated from the same powder by conventional powder metallurgy techniques.
To this end, and in accordance with the principles of the present invention, there is provided a stator core having an annular yoke portion comprising a compacted iron-based powder having individual particles substantially isolated by a dielectric layer. The stator core may further include a plurality of integral poles that extend either radially outward from the exterior circumference of the yoke portion or radially inward from an interior circumference of the yoke portion. The stator cores are formed by electromagnetic pressing an iron-based powder coated with a dielectric material to a density of at least about 98% of the theoretical density.
These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof.